Determination of sand content in digestate

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The biological treatment of separate collected waste becomes more and more important. The composting is still the dominant method of treating this waste, but the anaerobic alternative, the anaerobic digestion, plays an increasing part. In Germany already 15% of the whole capacity of biological treatment plants for separate collected biowaste are of anaerobic digestion (Kern, 1998), because of its affirmative energy balance and the neutrality regarding the greenhouse effect it is going to be more and more used. In the planning phase the danger coming from the mineral fraction, which is part of organic waste collected from households, is often overlooked. The mineral particles do not disturb the anaerobic process itself but they can damage or yet destroy parts of the technical constructions as pumps, tubes or valves.

The amount of the mineral fraction varies strongly, depending on the collection system (size of bins), on the collection area (rural or urban) and on the soil conditions in the region (e.g. sandy soil). Important is how much garden waste is collected for the anaerobic digestion and how much sand or other mineral fraction is adhered to the garden waste. To calculate a sand separator in the planning phase of a new plant, or for to retrofit one at an existing plant, the amount and the composition of the mineral fraction in the biowaste is very important.

At the moment there is no standard for the analysis of the mineral content in biowaste. The relevant literature and standards only describe the analysis of sand and mineral fraction in soil. The Institute of Waste Technology and Environmental Control (IfAU), at the University of Applied Sciences Braunschweig/Wolfenbüttel, compared three methods to determine the mineral content in biowaste Graul, 1999; Pelz, 1998). The two methods which were developed by IfAU and the third which is a modified one according to VDLUFA has been compared and evaluated concerning its range of use.

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